Technical Abstract: The U.S. National Plant Germplasm System (NPGS) maintains over 40,000 sorghum accessions, which is too large to adequately screen at one time. The objectives of this research were to establish an initial core collection and use clustering techniques to refine one sub-set of that core. Because much of the total sorghum collection lacks complete descriptor or other usable data, the collection was broken out into its respective country of origin and random selecton of 10% of the accessions from each country was used to develop the core. Based on these criteria, a core collection of 3011 accessions representing 77 different countries was developed. A more complete phenotypic dataset representing the Sudan allowed for the development of a refined subset of the core collection from this country. The core from Sudan is made up of 352 accessions that represent 13.8% of the total Sudan collection. This core was then evaluated for imbibitional high temperature sensitivity, leaf high temperature sensitivity, and acquired thermotolerance. The results show that these assay systems evaluate distinct aspects of the sorghums' metabolism and that each assay provides unique information about the relative heat tolerance of the sorghum. The impact of the sorghum core collection a this time is unknown. Theoretically, the core should provide a road map from which scientists can map genetic diversity of sorghum and further enhance the capabilities to isolate and clone genes of importance in the future.